Gaseous discharge device



United States Patent ()1 "ce 2,891,188 GASEOUS DISCHARGE DEVICE Vivian'L. Holdaway, Plainfield, N.J., assignor to Bell Telephone Laboratories,Incorporated, New York, N.Y., a corporation of New York 1 ApplicationMarch 24, 1955, Serial No. 496,431

3 Claims. (Cl. 313-451) This invention relates to gaseous dischargedevices, and more particularly to such devices especially suitable forusein switching networks. 1

In gaseous discharge switching networks, of the type described inBruce-Straube Patent 2,684,405, July 20, 1954, two desirablecharacteristics are a high breakdown voltage, so that the margin betweena devices breakdown and sustaining voltages is high, and stability ofbreakdown voltage. In order to attain a constant breakdown voltagedespite variations in spacing between different devices in the networkand variations in the gas density in a single device due to operationthereof, it has been proposed to operate such devices at the Paschenminimum of the voltage breakdown-pressure times distance curve; thePaschen minimum, or the pd minimum of the Paschen curve as it issometimes called, is the minimum breakdown voltage that can beXappliedto effect a discharge in the device. At the Paschen minimum the pd curvehas a small slope so that slight variations in the gas density in theanode-'to-cathode' gap will not afiect the value of breakdown voltage.Thus stability of breakdown voltage can be readily attained withinsatisfactory limits.

However, as stated above, this is the minimum breakdown voltage that canbe applied to the device and accordingly the breakdown voltage stabilityhas been attained by sacrificing another desirablecharacteristic foroperation in gaseous discharge device switching networks,- namely, ahigh breakdown voltage. Accordingly, it has been proposed also tooperate the device with a larger pd, and specifically with a larger maingap spacing, in order to increase the breakdown voltage. However, atthese other points on the pd curve, slight variations in spacing betweendevices or in the density in a given device, due to a prior discharge inthe device, give rise to large variations in the breakdown potentialrequisite 'to fire the tube, as the slope of the curve is large at theseother points.

It is possible to determine the initial spacing between the anode andcathode very accurately for a given desired breakdown voltage. This maybe attained by utilizing a wire anode positioned with its tip or endopposite to and facing a hollow cathode, and ionically bombarding theend of the anode to etch it away while maintaining a large dischargebetween the cathode and an auxiliary aging anode in the device. Thisetching is periodically interrupted for measurement of the breakdownvoltage of the main gap to determine if the anode-to-cat-hode spacinghas been reduced to thedesired distance. This process and an apparatusfor automatically initiating and interrupting the etching discharge andmeasuring the main gap breakdown voltage are further described in Patent2,825,618, of A. D. White, issued March 4, 1958.

The anode-to-cathode spacing may also be initially determined during themanufacture of the device in other ways, known inthe art, such as bymechanical jigging and optical measurements. Advantageously, the anode-2,891,188 Patented June 16, 1959 pressure are chosen to attain a stablenegative resistance characteristic at the current and frequency rangesat which it is intended to employ the device in a particular switchingnetwork, as further disclosed in Patent 2,804,565, of M. A. Townsend,issued August 27, 1957.

However, even though the initial spacings between the main anode andcathode of the various devices in the network be very accuratelydetermined to attain the same breakdown voltage for all the devices,nevertheless various devices in the network may, at any given instant,

: have a lower breakdown voltage than other devices. This arises due tothe localized heating of the cathode of the device by the discharge.Heating of the cathode in turn heats the gas immediately adjacent to it,and thus within the cathode-to-anode gap, thereby reducing the gasdensity. When the density of the gas in the gap is changed, thebreakdown voltage of the gap varies in the same way as if instead of thedensity the distance between the electrodes had been changed, and in thesame proportion. Thus, heating of the gas in the gap by the dischargecauses a reduction of the breakdown voltage required for that. device.As this heating occurs while the device has a discharge sustaining init, it has generally not been considered serious. However, in switchingnetworks employing gaseous discharge devices, a device may be utilizedin a subsequent connection or path through the network immediately uponextinction of a prior discharge in the device'and before the gas has hadan opportunity to cool and the breakdown voltage of the tube return toits original and higher value.

If "a new connection is to be set up through a switchin-g network aftera prior connection has been interrupted but before the devices employedin that connection have had'an opportunity to cool, these devices,having lower breakdown voltages, will be preferred also for the nextconnection. What will then occur is that a rut will e'Xist'in thenetwork in which connections will always prefer to go through a few ofthe devices in the network, because of their lower breakdown potentials,rather than even through all of the devices on a statistical basis. Thiswill causethedevices of the network not to be used equally, with theresult that a few-devices: will be quickly burned out while the otherdevices are not employed.

A further serious difiiculty that then arises when the breakdown voltageof devices employed immediately priorly on other paths is lower is thepossibility that false paths may result from the degraded mar ins in thenetwork. In order to assure against these false paths all components andvoltages utilized in the network are maintained to close tolerances sothat the maximum pileup will not cause improper voltages to appearacross devices, which voltages approach the breakdown voltage of adevice not to be broken down in setting up a connection, such as adevice having one terminal connected to an existing connection throughthe network. However, if the breakdown voltage of a device is decreased,as described above, and a maximum adverse culmination of tolerancesoccurs, false paths may result. This can be guarded against byincreasing the margins, decreasing the allowable tolerances, anddecreasing the number of stages, but, as is readily apparent, each ofthese would result in considerable sacrifice in economy and efiiciencytO-cathode spacing, the shape of the cathode,'and the gas of operationof the network.

It is an object of this invention to provide an improved gaseousdischarge device and specifically such an improved device for operationat other than the Paschen minimum.

It is a further object of this invention to maintain the breakdownvoltage of a gaseous discharge device constant regardless of the prioroperation of the device.

It is a still further object of thisinvention to prevent one or a numberof discharge devices employed in a switching network being utilizedexcessively in connections set up through that network. Thus it is anobject of this invention to enable connections through a switchingnetwork employing gaseousdischarge devices to utilize the variousdevices available for any path between two points in that networkevenly.

It is a still further object of this invention to provide a gaseousdischarge device in which the: breakdown voltage is independent of theprevious conducting or nonconducting state of the device.

These and other objects of this invention are attained in one specificillustrative embodiment wherein a gaseous discharge device comprises ahollow cathode and an anode positioned opposite thereto and defining agap therewith, the cathode, gap, and pressure being advantageously suchthat the device has a stable and usable negative resistancecharacteristic, as disclosed in the above-mentioned Townsend patent. Thecathode is mounted on a bimetallic support which is constructed so thaton heating of the cathode, by the discharge, the variations in the gasdensity in the cathode-to-anode gap will @be compensated for by motionof the cathode away from the anode, thereby increasing the size of thegap. In this manner the breakdown voltage is maintained constant as theproduct pd remains constant.

The heating of the cathode and therefore the decrease in gas density atthe cathode can be determined for a given discharge current and theincrease in spacing re-. quired can then be calculated. Accordingly, thethermally responsive element supporting the cathode can readily beconstructed to have a thermal characteristic so that it will provideprecisely thisamount of motion of the cathode away from the anode forthis decrease in gas density at thecathode or may, if desired, bedesigned to overcompensate slightly.

It is a feature of this invention that the cathode of a gaseousdischarge device be supported by a thermally responsive element soconstructed that motion of the cathode directly compensates forvariation in the gas density adjacent the cathode to prevent decrease inthe breakdown voltage of the device due to decrease in the gas densityadjacent the cathode.

It is another feature of this invention that the breakdown voltage of agaseous discharge device operated at other than thepd minimum of thePaschen curve be maintained constant with change of density in theanode, to-cathode gap due to heating of the cathode by the dish r ym nns 1 c d Q 1 b me l ic s pv having a thermal characteristic such thatthe motion of I the cathode with relation to the anode exactly compen-vsates for the change in density in the anode-to-cathode gap.

A complete understanding of this invention and of these and otherfeatures thereof may be gained from consideration of the followingdetailed description and the accompanying drawing, in which:

Fig. 1 is a Paschen curve of breakdown voltage as a function of thepressure-distance product for an exemplary discharge device; and

Fig. 2 is a perspective view of one illustrative embodiment of thisinvention, a portion of the envelope being broken away to show theinternal elements more clearly.

Fig. l is a typical plot of voltage breakdown V as a function of thepressure-distance product, pd. The solid portion of the curve can bedetermined experimentally quite readily by maintaining the gas pressureconstant and varying the spacing between the cathode and the anode. Atsome pressure and spacing corresponding to point on Fig. 1., breakdownoccurs when the applied voltage is high enough so that each electroncrossing the gap produces sufficient ionizations and'excitation in thegap to release another electron at the Cathode.

As the anode is moved closer to the cathode, -.a givfin appliedvoltageproduces a larger electric fieldand the efficiency of theionization increases. .The necessary voltage for breakdown consequentlydecreases. As the spacing is decreased further, a minimum breakdownvoltage is realized at the point 11, which is often referred to as thePaschen or pd minimum. At this point the spacing between the electrodesis still large enough to enable a sufficient number of ionizingcollisions in the gap and the electric field at the cathode is high.

As the spacingv is further decreased, the number of collisions anelectron can make in passing from the cathode to the anode become lowenough to outweigh the aiding effect of the higher field strength at thecathode, resulting in an increase in the breakdown voltage This isindicatedby the dashed portion 12 of the curve. This portion of thecurve is difiicult to find experimentally as the electrons will preferto traverse a longer path from the cathode to the back of the anode orto the anode support wire, so that the actual distance d that isinvolved is, not definite. ,It can be measured however by usinggeometries which restrict these longer paths. With usual geometries, theactual breakdown voltage at these low values of pd is indicated by thedotted portion 13 of the curve and does not rise as greatly as theoryindicates (due to these longer paths around the anode).

The breakdown voltage is also dependent on the particular gas and thecathode material, as well as on the pressure-distance product. In theparticular devices for which this curve was plotted, as described morefully below, a molybdenum cathode was employed and the device was filledwith neon.

It canreadily be seen from Fig. 1 that one method of obtaining fairlyconstant breakdown voltage is to operate the device at the pd minimumpoint 11, as decrease. in density, due to heating of the cathode, willcause'the breakdown voltage to follow the dotted portion 13 of thecurve. However, this approach to the problem of maintaining thebreakdown voltage constant requires that the breakdown voltage be aminimum and this may be very undesirable. It is therefore greatly to bepreferred that the device be operated at some point to the right of thepd minimum, such as the point 15, with a breakdown voltage considerablyhigher than that at the Paschen minimum; in the embodiments for whichthis plot is descriptive the breakdown voltage at the Paschen minimum isvolts and the breakdown voltage desired atpoint 15 is 220 volts or anincrease of 30 volts.

Now, however, a decrease in the gas density adjacent the cathode will ineflect cause the pa? product to decrease so that the breakdown voltagemay be at a lower point on curve 10, such as point 16. Here thebreakdown voltage is only 200 volts, a drop of 20 volts from thebreakdown voltage of the tube when cold. it In accordance with thisinvention, a gaseous discharge deviceis provided which maintains thepressure-distance product constant so that the breakdown point of thedevice is always at the design point 15 regardless of the, prior operateor non-operate condition of the device. One specific illustrativeembodiment of this invention is depicted in'Fig. 2 and comprises a wireanode 20 supported within a glass envelope 21 by a support lead 22. Theanodeis positioned with its tip or end pointing towards a hollow cathode24, which may be a helically coiled cathode as depicted, though othertypes of hollow cathodes may advantageously be employed. The dimen:sions of cathode 24, the spacing between the cathode and the anode, andthe gas pressure may advantageously be such that .theldevi'ce has astable negative resistance characteristic over a usable current andfrequency range, as further taught in Patent 2,804,565, of M.A.';Townsend, issued August 27, 1957. The various types of cathodesdisclosed in this Townsend patent may also all be employed, but thisinvention is not to be considered as limited to these or any particularcathode conf guration. The baseof 'thecathode is secured, as by welding,near one end vof a bimetallicstrip comprised ,of plates 26 and 27. Ifdesired, one end of the cathode coil may depend from the cathode and besecured to the bimetallic strip. Heat conduction from the cathode to thebimetallic strip is improved however by assuring a good thermal paththerebetween. The bimetallic strip is secured near its other end, as bywelding, to a cathode support lead 28. The cathode 24 and support lead28 may be secured to the same or opposite sides of the bimetallic strip.In accordance with an aspect of this invention, the two plates 26 and 27which comprise the bimetallic strip or element are so arranged thatheating of the plates, due to heating of the cathode 24, will cause theplates to bend, moving the cathode 24 farther away from the anode 20. Inone specific illustrative embodiment, the plate 26 Was a .010 x .050 x.187 inch molybdenum plate and plate 27 was a nickel plate with the samedimensions.

The operation of the bimetallic support to compensate for the change indensity can readily be seen from consideration of a specificillustrative example. In this specific example the anode 20 was a .005inch molybdenum wire mounted with its tip initially, i.e., in the coldcondition, .012 inch from the hollow cathode 24, which was ofmolybdenum. The envelope was filled with neon at 100 millimeters ofpressure. In the cold state the temperature of both the cathode 24 andthe envelope 21 was 25 C.

in a gaseous discharge device of this type in accordance with the priorart and without a compensatory cathode mount, the device, immediatelyafter operation at a discharge current of .010 ampere would have thefollowing characteristics. The cathode temperature would be 250 C. andthe average envelope temperature 75 C. causing the gas density at thecathode to decrease. The pd product under these conditions correspondsto the point 16 on curve and a breakdown voltage of 200 volts.

In gaseous discharge devices in accordance with this invention, however,having compensatory cathode mounts the cathode is arranged to move awayfrom the anode to increase the gap distance d an amount just suflicientto keep the pd product constant. In one specific embodiment in which thepd product remained constant, the bimetallic support provided a 6 milmovement when the tube had been operated at 0.010 ampere.

While a specific type of thermal compensating support using particularmetals has been depicted and described, it is to be understood thatvarious other types of such supports may be utilized. Further, theamount of motion required exactly to compensate for the change indensity in the anode-to-cathode gap will of course depend on thespecific device, the operating current, the original gas pressure, andthe increase in temperature of the cathode due to the discharge.

In the above described embodiment, the bimetallic element has beenarranged exactly to compensate for the change in density so that thebreakdown voltage is maintained constant. In certain applications it maybe desirable, however, slightly to overcompensate, thus giving aninitial preference in the network to employing devices not immediatelypriorly employed in a connection through the network. Further, arrangingthe various devices so as to be slightly overcompensated will enable thedesign to be less critical. Conversely, in certain arrangements it maybe satisfactory if the compensation aiforded by the thermal mounting ofthe cathode be somewhat less than required for exact compensation.

In certain applications very adverse ambient tempera ture conditions maybe encountered. If the ambient temperature increases appreciably, therewill be motion of the cathode, because of its thermally responsivesupport, which is due to this ambient temperature increase and notcaused by a local temperature variation due to the glow discharge at thecathode. Variations in ambient temperature, however, do not affect thegas density and thus do not afiect the breakdown voltage. Accordingly,if severe ambient temperature variations are to be encountered, theanode may be also supported by a thermally responsive element, identicalwith that supporting the cathode, so that the anode moves to follow theoath ode and prevent ambient temperature changes aifecting the desiredbreakdown voltage characteristic.

Accordingly, it is to be understood that the abovedescribed arrangementsare illustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

1. A glow discharge device comprising an envelope, a gaseous filling insaid envelope at a predetermined pressure, a hollow cathode within saidenvelope, a wire anode positioned with its end opposite to and directedtowards the hollow portion of said hollow cathode and defining therewitha gap having an initial predetermined spacing, whereby said dischargedevice has initially a predetermined breakdown voltage, and means formaintaining said breakdown voltage constant with change in density ofsaid gaseous filling in said gap due to .heating of said cathode by adischarge, said means comprising a bimetallic support for said cathodehaving a thermal characteristic such that the increase of spacing ofsaid gap from said initial predetermined spacing exactly compensates forthe decrease in density of said gaseous filling in said gap from saidpredetermined filling.

2. A glow discharge device for operation in a region where the breakdownvoltage changes with density comprising an envelope, a gaseous fillingwithin said envelope, an anode, a hollow cathode opposite said anode andde fining a gap therewith, said gap having a predetermined breakdownvoltage, and means for preventing substantial decrease in said breakdownvoltage on change in the gas density in said gap due to heating of saidcathode by the glow discharge, said means comprising a bimetallicelement supporting said cathode for motion towards and away from saidanode and having a thermal characteristic such that said cathode movesaway from said anode on heating of said cathode an amount compensatingfor the change in gas density in said gap on heating of said cathode.

3. A glow discharge device for operation at other than the pressuredistance minimum of the Paschen curve comprising an envelope, a gaseousfilling within said envelope, an anode, a cathode capable of maintaininga large glow discharge opposite said anode and defining a gap therewith,said gap having a predetermined breakdown voltage, and means forpreventing said breakdown voltage decreasing on change in the gasdensity in said gap due to heating of said cathode by said glowdischarge, said means comprising a bimetallic support for said cathodehaving a thermal characteristic such that the increase in the gapspacing on heating of said cathode is at least sufficient to prevent adecrease in the breakdown voltage of said gap.

References Cited in the file of this patent UNITED STATES PATENTS1,031,796 Jackson July 9, 1912 1,537,680 Klahre May 12, 1925 1,617,065Lorenz Feb. 8, 1927 1,640,450 lhln Aug. 30, 1927 2,380,496 Beard July31, 1945

